1. Field of the Invention
The present invention relates to resin gears applicable to a drive transmission mechanism in, for example, copying machines, facsimile machines, or printers. The present invention also relates to a developing unit, a photoconductor drum unit, an image forming apparatus or an image reading apparatus having the resin gears.
2. Description of the Related Art
In a drive transmission mechanism for various machines, such as a drive transmission mechanism for copying machines, facsimile machines, or printers, a drive force from a drive source such as a motor is transmitted via a gear. Such gears for drive transmission are manufactured by machining metal. However, gears formed of resin are used to take advantage of cost and weight reduction.
For example, in the first related art, the gear is formed with voids on both ends of the teeth in the direction of the width of the teeth for reducing the thickness of the teeth by a predetermined depth toward the direction of the width of the teeth, and the thickness of the teeth of portions in which the voids are formed is larger than portions without being formed with the voids considering the contraction difference after injection molding. In the second related art, when manufacturing resin worm wheels, the shape of the teeth is finished by cutting work after having molded a formed and molded member by a metal mold. In the third related art, a backlash-less gear formed of resin into a simple structure without increasing the width of the teeth is used.
In the case of the resin gears in the related art, in the case of a spur wheel, for example, the tooth thickness Hb at a tooth root is larger than the tooth thickness Ht of a tooth tip of a tooth T as shown in FIG. 5. In a case of forming a resin molding process, when the thickness of the resin varies, a portion having a larger thickness has a larger contraction percentage when being cooled and cured, and hence there arises a problem in that the designed dimensions are not achieved due to uneven contraction of the entire gear.
In the fourth related art, a plurality of ribs are radially arranged at the roots of the teeth integrally with webs formed into a disc shape for securing necessary rigidity. In the fifth related art, teeth with a twisted teeth trace are formed on the outer peripheral side of an annular portion, and a plurality of reinforcing ribs are arranged radially on the inner side of the annular portion. In the sixth related art, a boss formed at a center portion, a rim formed concentrically with the boss in the outer peripheral direction, and a thin web connected integrally between the boss and the rim are provided, and the web is formed with ribs formed integrally and radially at positions corresponding to all the teeth.
The resin gear in the related arts as described above is provided with the plurality of radially formed reinforcing ribs on the inner side thereof, and in such a resin gear, reinforcing ribs R′ are connected to, and integrally formed at, positions corresponding to tooth roots of teeth T′ (portions surrounded with circles of dotted lines) as shown in FIG. 10. In this shape, resin portions having a larger thickness are formed continuously from the teeth T′ to the reinforcing ribs R′. Therefore, when performing a resin molding process, since the contraction percentage of the resin portions having a larger thickness is larger, the entire gear is unevenly contracted when cooling and curing the resin, whereby the gear cannot be formed to have designed dimensions.
In particular, the gears used in the drive transmission mechanism for copying machines, facsimile machines or printers, when the dimensional accuracy is not high, may cause errors in a paper feeding operation or in an image forming process, so that deterioration of images occurs.